1. Field of Invention
The present invention provides a method of producing particles using supercritical fluid, an apparatus for carrying out the method and particles formed thereby.
2. Description of Related Art
Several processing techniques utilize the enhanced mass-transfer properties and benign nature of supercritical or near-critical fluid or compressed gas (hereinafter collectively referred to and defined as “supercritical fluid” or “SCF”) to produce composite or single-material particles. One such method or technique, which is sometimes referred to as the conventional Supercritical Anti-Solvent (SAS) or Gas Anti-solvent (GAS) processing technique, involves the addition of a supercritical fluid to a solution containing a solute dissolved in an organic solvent. The supercritical fluid acts as an anti-solvent in that it precipitates the solute in the form of particles that are suspended in the supercritical fluid.
There are several disadvantages of conventional SAS method including particle agglomeration (“bridging”) caused by particle contacts during nucleation, aggregation due to plasticization of some materials (for example polymers) in SCF and also insufficient control of particle size distribution. These disadvantages can be associated with the fundamental mechanism of precipitation in SCF. It is therefore desirable to provide an additional control over particle formation by introducing an additional solvent (the “second solvent”) that influences the precipitation process. For example, precipitation of the solute in the presence of the second solvent can influence the particle growth characteristics and mass transfer rates therefore causing alteration in the particle precipitation kinetics. This in turn can lead to precipitation of particles with differing physical properties such as crystallinity, crystal shape, uniformity and size.
In addition, the conventional SAS method is typically carried out as a batch process. It is very difficult, if possible at all, to scale up a conventional SAS batch method as a continuous operation on the industrial scale. Therefore addition of a solvent can allow the product to be transported out the pressurized precipitation vessel, thus allowing for a continuous sample collection.